As the power transmission gear of construction equipment, etc. the following systems have heretofore been used.
1. Mechanical power transmission system PA0 2. Hydraulic power transmission system (HST) PA0 3. Mechanical-hydraulic power transmission system (HMT)
In the mechanical-hydraulic power transmission system (HMT), part of the power is transmitted mechanically and the rest of the power is transmitted hydraulically; in other words, a power division type hydraulic transmission system (hydraulic-mechanical power transmission), among which an output division type as shown in FIG. 1 is well known.
However, in the above-mentioned prior art transmission gears, the mechanical power transmission system is excellent in power transmission efficiency, but poor in the control of change-over from forward running to reversing and vice versa. The hydraulic power transmission system (Refer to FIG. 2) is capable of effecting control of change-over from forward rotation to reversing and vice versa, by varying the tilting angle of swash plates of a variable displacement hydraulic pump 51 and a variable displacement hydraulic motor 52. The system and therefore can be operated easily, and is also capable of changing the rotational speed of the output shaft by engagement between gears 53 and 54, engagement between gears 55 and 56 and change-over between clutches 57 and 58. However, such a system is disadvantageous in that when the output shaft is rotating at high speeds the tilting angle of the swash plate of the pump 51 is increased to increase the discharge flow-rate of fluid thus increasing the pressure loss, while the tilting angle of the swash plate of the motor 52 is reduced thus causing a lowering in efficiency. For this reason, when the output shaft of the mechanical-hydraulic power transmission system (Refer to FIG. 1) is rotating at high speeds mechanical power transmission is effected so as to avoid lowering in efficiency. While when the output shaft is rotating at low speeds, hydraulic power transmission is effected so as to facilitate control of change-over from forward rotation to reversing and vice versa. This type of system has been used in various fields.
In FIG. 1, an input shaft 62 is connected to a prime mover 61, and an intermediate portion of the input shaft 62 has a gear 64 fixedly secured thereto and which meshes with a hydraulic pump driving gear 63. Further, a sun gear 65 of a planetary gear device A is fixedly secured to one end of the input shaft 62. The hydraulic pump driving gear 63 is arranged to drive a variable displacement type pump 66 of a hydraulic transmission gear B provided on one side of the input shaft of driving gear 63. This variable displacement type pump 66 is arranged to actuate a fixed displacement motor 67 mounted in juxtaposition thereto and parallel with the axis of the input shaft 62. This fixed displacement motor 67 has an output shaft provided with a gear 68 to which the power developed by the motor 67 is transmitted. The gear 68 is arranged to rotatively drive an internal gear 69-1 of the planetary gear device A through a gear 69 which meshes therewith. A planetary gear 70 is provided between the internal gear 69-1 and the sun gear 65. The planetary gear 70 has a shaft supporting frame 71 which is connected to an output shaft 72. The arrangement is made such that revolution of the planetary gear 70 around the sun gear 65; that is, rotational motion of the shaft supporting frame 71 can be transmitted to a starting wheel 73 mounted on the body of a construction vehicle. When the variable displacement type pump 66 is at its neutral position, the hydraulic transmission gear B conducts only braking action and the internal gear 69-1 is fixed. Therefore the planetary gear device A serves as a mechanical planetary reduction gear. Speed control of the output shaft 72 is effected by a speed control means, not shown, such as a governor or the like provided in the prime mover 61. Further, when the variable displacement type pump 66 is actuated, part of the power developed by the prime mover is transmitted in turn through transmission elements 64, 63, 66, 67, 68, 69 to the planetary gear 70, while the rest of the power is mechanically transmitted in turn through transmission elements 62, 65 to the planetary gear 70.
The above-mentioned prior art mechanical-hydraulic power transmission system has the following disadvantages. Since the fixed displacement motor 67 is arranged to drive the internal gear 69-1, in order to change the number of revolutions of the output shaft from forward rotation to reversing, it is essential to use a double-discharge, variable displacement pump 66 as the hydraulic pump and to rotate the internal gear 69-1 at a considerably high speed when the number of revolutions of the output shaft 72 has reached a predetermined value. Therefore, the ratio of displacement between the hydraulic motor and the hydraulic pump must be set at a very big value. In practice, selection of a hydraulic pump and a hydraulic motor which meet such conditions is extremely difficult. Further, such a prior art mechanical-hydraulic transmission gear system is disadvantageous in that, since the hydraulic pump, the hydraulic motor and the planetary gear device constituting the hydraulic transmission gear B are juxtaposed in the axial direction of the input shaft 62, the whole system becomes large in size. Further, in the case where the fluid circuit is changed over to supply the fluid under pressure discharged by the hydraulic pump to another hydraulic actuator such as an implement of the construction equipment, the power is always partially transmitted to the output shaft 72. Even if the output shaft 72 is fixedly secured by means of a brake or the like, the fixed displacement type motor 67 is rotated by the power transmitted from the planetary gear 70 through the internal gear 69-1 and the gear 68 thereto, thus causing rotational losses or power losses.